U.S. Homeland Security Secretary Janet Napolitano says authorities are becoming more concerned about American citizens or legal residents becoming involved in terrorist plots against the United States.

Speaking Sunday in Washington to a gathering of U.S. state governors, Napolitano said law enforcement officials do not have good methods for preventing people from becoming violent extremists.

In one recent case, five young Pakistani men who lived in the northern Virginia suburbs of Washington, D.C., traveled to Pakistan where they allegedly attempted to join militant Islamist groups.

In a separate incident, an Afghan-born resident of the western state of Colorado was arrested last year and charged with conspiring to use weapons of mass destruction.

Officials at the meeting of U.S. governors endorsed outreach efforts by law enforcement agencies to Muslim communities.